Method of pickling and chemically milling zirconium and zirconium alloys



Annapolis, Md., assignors to Martin Marietta Corporation, New York,N.Y., a corporation of Maryland No Drawing. Filed Feb. 14, 1963, Ser.No. 258,638 4 Claims. (Cl. 252-793) This invention relates to thesurface treatment of zirconium and zirconium alloys to remove metal andscale therefrom. More particularly, the invention concerns a novelmethod for controlled etching, chemical milling, or pickling ofzirconium and its alloys with an aqueous solution of an alkali metalfluoride.

The processing of zirconium and its alloys is of importance inconnection with the operation of nuclear reactors in which they serve ascontainers for the nuclear fuel. An alloy of zirconium which isextensively used for this purpose is Zircaloy-2, which has thecomposition Sn 1.5%, Fe 0.12%, Cr 0.10%, Ni 0.05%, balance Zr, all byweight.

It is known in the prior art to employ as etchants for zirconium alloys,hydrofluoric acid, or aqueous solutions of ammonium bifluoride, orcombinations of hydrofluoric acid and nitric acid.

These prior art etchants and milling agents have several disadvantages.Hyd-rofluoric acid baths are highly toxic and corrosive and createsafety problems for workers. The hydrofluoric acid-nitric acid solutionis highly corrosive and requires costly resistant equipment in theplant. Moreover, in typical hydrofluoric acidnitric acid combinations,such as 39% HNO -3.8% HF, the acidity is so high that the bath is toofast-acting to serve successfully as an agent for chemical milling andpolishing. In order to reduce the etching rate and to obtain bettercontrol, it has been proposed to employ aqueous solutions of ammoniumbifluoride, NI-I F.HF, either in combination with nitric acid, orfollowed by a nitric acid rinse. Such baths can contain a nonionicsurfactant such as an alkyl polyoxyethylene glycol amide. However,bifluoride salt baths of themselves are not entirely satisfactory forchemical milling or etching of zirconium and its alloys because it iswell known that solutions of fluoride salts attack zirconium. Hencewhere fluoride salts per so are employed, the operation must be confinedto a comparatively narrow range of process temperature and fluorideconcentration. There has therefore existed a need for an aqeous bathwhich would provide the advantages of low toxicity and freedom fromcorrosion characteristic of fluoride salt baths while at the same timeproviding a wider latitude of processing temperatures, fluorideconcentrations, absence of free nitric acid and/or hydrofluoric acid,and a readily controllable rate of attack on the metal.

In accordance with the invention, it has been found that aforementioneddisadvantages are eliminated and the desirable characteristics ofsafety, noncorrosiveness, controllable rate of attack on metal, andwider range of concentrations and temperatures are attained by employingan aqueous solution of an alkali metal fluoride which contains inaddition, sulfamic acid NH SO OH, or an alkali metal salt of sulfamicacid. The alkali metal salts of sulfamic acid which are suitable includepotassium,

United States Patent 0 "ice sodium, and ammonium sulfamates. The alkalimetal fluorides which may be employed include water soluble fluorides ofpotassium, sodium, and ammonium. Furthermore, double fluoride salts ofalkali metals, such as potassium, sodium, and ammonium bifluoride mayalso be employed together with sulfamic acid and its alkali metal salts,in concentrations equivalent to those employed for the normal fluoridesalts.

The precise role of the sulfamic acid is not known, but it appears toexert a regulating as well as a potentiating action upon the fluoridesalt which permits a relatively slow rate of attack upon the zirconiummetal or zirconium alloy, thus making it easier to maintain closetolerances, while at the same time the concentration range of thefluoride salt may be varied over a wider range according to theobjective desired, whether etching, chemical milling or pickling, thanwas possible with hitherto employed preparations.

The proportion by weight of sulfamic acid or its salt to fluoride saltis not critical, but in general the baths of the invention contain agreater proportion of the sulfamic acid compound.

Thus, the amount of sulfamic acid present in the bath willadvantageously range from about 5% to about 25% by weight, or an amountof alkali metal sulfamate sufficient to furnish the equivalent range ofsulfamic acid.

The amount of alkali metal fluoride employed will advantageously rangebetween about 1% and about 6% by weight, preferably between about 2% andabout 5%.

A preferred bath of the invention comprises an aqueous solution of about10% sulfamic acid and about 4% potassium fluoride, by weight.

The optimum acidity of such milling and polishing baths is related tothe fluoride salt used and is about 2.75 for NaF and about 3.75 for KFand NH F. At higher acidities, below pH 2.75 or even pH 2.5, theactivity of the bath increases so that it becomes more suitable foretching and pickling applications. At pH values above about 4, the bathmay exhibit a tendency to produce pitting. The milling baths in therange of about 2.75 to 3.75 exhibit favorable rates of metal removal,producing a rel atively slow rate of attack upon the metal and making iteasier to maintain close tolerances. At the same time the surfacematerial is removed evenly, but at sufficiently rapid rates to providepractical and effective milling.

As indicated previously the weight ratio of sulfamic acid to fluoridesalt is not critical, but it will advantageously be maintained betweenabout 4:1 and about 2:1.

The general method of treatment of the zirconium or zirconium alloycomprises the steps of first subjecting the metal to a degreasingtreatment in accordance with conventional procedures, as for example,vapor degreasing in stabilized perchlorethylene at elevated temperature.The metal is then further cleaned with an alkaline solution, such forexample an aqueous solution of trisodium phosphate, also at elevatedtemperature. The metal is then thoroughly rinsed, preferably first withtap water, and then with demineralized water.

The etching, pickling or milling step is carried out by immersing themetal in a suitable solution of sulfamic acid or an alkali metalsulfamate, and an alkali metal fluoride, the concentrations of theingredient being determined by the type of treatment. The time ofimmersion will depend upon the amount of metal to be removed,

but may range up to about minutes. Thus, the milling' rate of a bath atpH 2.75 is approximately 0.3 mil per minute exposure. The temperature ofthe bath may range between about 18 C. and about 50 C., but 35 C. ispreferred.

The treatment step is accompanied by the formation of a grayish-blacksmut, which builds with increasing rapidity after about 3 or 4 minutesimmersion in the bath, decreasing the baths reactivity. This smut may beremoved by rinsing the metal thoroughly in tap water, followed by briefimmersion in a solution of nitric acid of concentration 50% HN0 byweight until the smut is dissolved. At times mere dipping in nitric acidsolution is suflicient.

If the removal of a considerable amount of surface metal is required, itmay be necessary to expose the work to alternate milling treatment andsmut removal until the desired condition is achieved. The treatment iscarried out at room temperature.

Following the smut removal step, the metal is rinsed thoroughly, firstwith tap water, then with demineralized water, and is finally dried, forexample, by means of a hot air blast.

The following examples illustrate preferred procedures of the invention,but are not to be regarded as limiting:

Example I An aqueous chemical milling bath was prepared by dissolving inwater 10.6% by weight of sulfamic acid, and 4.2% by weight of potassiumfluoride. The bath temperature was adjusted to 35 C. A rod of Zircaloy-Zwas subjected to vapor degreasing in stabilized perchlorethylene at 249F. for minutes. The metal was further cleaned by immersion into asolution of trisodium phosphate at 180 F. for 5 minutes, rinsedthoroughly, first with tap water and then with demineralized water. Themetal alloy was immersed in the aforementioned bath for 3 minutes,removed, rinsed thoroughly in tap water, and then subjected to smutremoval by immersion in a 50% HNO solution for 2 minutes at roomtemperature. The article was then rinsed thoroughly, first with tapwater, then with demineralized water, and finally dried by means of ahot air blast to give an evenly milled, smooth surface.

In this way, an average of 0.80 mil of the Zircaloy-Z was milled away asa result of treating six work pieces.

Also, six like treatments were performed when the Zircaloy-Z wasimmersed in the milling bath for 1 minute and 5 minutes. In the firstinstance 0.32 mil of the alloy and, in the second instance, 1.07 mils ofthe alloy were milled away.

A number of other baths were formulated with sodium, potassium orammonium sulfamate (NaOSO NH KOSO NH NH OSO NH in various combinationswith the acid salts of sodium, potassium or ammonium fluoride (NaF-HF,KF-HF, NH F-HF) in concentrations which were approximately equivalent tothe most effective sulfamic acid-potassium fluoride concentration set'out in the example above. Additionally, baths were formulated withsulfamic acid and fluorides other than potassium fluoride, i.e., sodiumfluoride, NaF, and ammonium fluoride, NH F, on an equivalent fluoridebasis.

Representative additional baths and milling rates on Zircaloy-2 are setout below. The methods of treatment and evaluation were those set out'in Example I. The average milling rates were determined by measuring theamount milled away after one minute, three minutesand five minutes.

Example II Bath: Percent Ammonium sulfamate 12 Ammonium bifluoride 8Sulfamic acid 3.5

Milling rate: About 0.3 mil/minute.

4 Example III Bath: Percent Potassium sulfamate 9.5 Ammonium bifluoride4 Sulfamic acid 3.5 Milling rate: About 0.4 mil/ minute.

Example IV Bath: Percent Sulfamic acid 10 Sodium fluoride 3 Millingrate: About 0.3 mil/minute.

Example V Bath: Percent Sulfamic acid 10 Ammonium fluoride 3 Millingrate: About 0.2 mil/minute.

The test result with these baths, using Zircaloy-2, were well defined.It appears that any combination of these ingredients which produces asolution whose pH is 4 or less, with no insolubles present, shows someactivity as a milling solution. The optimum acidity for chemical millingand polishing in these baths is in the pH 2.75 to 3.75 range. Thereactivity of the bath increases steadily to an exclusively etchingsolution as the pH decreases below 2.5. Above pH 4, there is a tendencyto pit. The baths in the range pH 2.75 to 3.75 are sufliciently slow toremove surface material evenly and sufliciently fast to be practical.The formation of a grayishablack smut, which can be removed in a nitricacid bath, is a milling rate control mechanism. If the removal of aconsiderable amount of surface material is required, it is necessary toexpose the work alternatively to the milling formulation and smutremovals as in the nitric acid bath. As pointed out above the millingrate of a bath at pH 2.75 in Example -I and certain of the other bathsis approximately 0.3 mil per minute exposure but the aforementioned smutbuilds up rapidly after about 4 or 5 minutes and the reactivitydecreases steadily until the smut is removed in the nitric acidsolution. A quick dip in the nitric acid may be all that isrequired toremove the smut.

In the claims, it will be understood alkali metal fluoride is inclusiveof both the normal salt and the double salt, known as the bifluoride,and that the quantities specified in the claims refer to the quantitiesof the normal salt, or equivalent quantities of the double salt.

The rates mentioned above may be contrasted with the rates of removal ofmetal from Zircaloy-2 in the nitric acid-hydrofluoric acid bathmentioned earlier, which is almost linear with time, and averages about1.4 mils per minute.

Clearly such a rapid rate of metal removal is more aptly designated asmetal etching and such an acid bath is more aptly designated as anetchant. It was found impossible to remove metal from zirconium andzirconium alloys to conform with close tolerances, such as required incertain classes of threads, when such etching baths were used. On theother hand, the desired tolerances were readily obtained in the millingbaths of this invention.

It will be apparent the foregoing details are given for illustrativepurposes and that many variations in these details may be made withoutdeparting from the spirit of the invention or its scope as defined inthe appended claims.

What is claimed is:

1. A controlled chemical milling process for the surface treatment ofzirconium and zirconium alloys to remove precise amounts of metal andscale therefrom which comprises immersing and chemically milling themetal to a desired smaller dimension in a treating bath consistingessentially of an aqueous solution containing between about 5% and about25% by weight of a member of the group consisting of sulfamic acid andits alkali metal salts and between about 1% and about 6% by weight of analkali metal fluoride, the amount of sulfamic acid present being inexcess of the amount of alkali metal fluoride.

2. The method of claim 1 in which the metal is further treated in anitric acid bath to remove smut formations.

3. The controlled chemical milling process for zirconium alloys toremove precise amounts of metal and scale therefrom which comprisesimmersing and chemically milling the metal to a desired smallerdimension in a treating bath consisting essentially of an aqueoussolution of a member of the group consisting of sulfamic acid and itsalkali metal salts and an alkali metal fluoride, said bath having a pHabove about 2.5 but not above a pH of 4.

4. Method for the controlled chemical milling of zirconium and zirconiumalloys comprising immersing and chemically milling the metal to adesired smaller dimen- References Cited by the Examiner UNITED STATESPATENTS 2,220,451 11/1940 Hunt 252-142 10 2,879,186 3/1959 Fischer252142 XR 3,033,795 5/1962 Brevik 252-793 XR LEON D. ROSDOL, PrimaryExaminer.

15 JULIUS GREENWALD, Examiner.

W. E. SCHULZ, Assistant Examiner.

1. A CONTROLLED CHEMICAL MILLING PROCESS FOR THE SURFACE TREATMENT OFZIRCONIUM AND ZIRCONIUM ALLOYS TO REMOVE PRECISE AMOUNTS OF METAL ANDSCALE THEREFROM WHICH COMPRISES IMMERSING AND CHEMICALLY MILLING THEMETAL TO A DESIRED SMALLER DIMENSION IN A TREATING BATH CONSISTINGESSENTIALLY OF AN AQUEOUS SOLUTION CONTAINING BETWEEN ABOUT 5% AND ABOUT25% BY WEIGHT OF A MEMBER OF THE GROUP CONSISTING OF SULFAMIC ACID ANDITS ALKALI METAL SALTS AND BETWEEN ABOUT 1% AND ABOUT 6% BY WEIGHT OF ANALKALI METAL FLUORIDE, THE AMOUNT OF SULFAMIC ACID PRESENT BEING INEXCESS OF THE AMOUNT OF ALKALI METAL FLUORIDE.